Tuesday, December 31, 2013

New computer model claims global warming decreases clouds

A new paper published in Nature claims global warming reduces low clouds, the opposite of what has been claimed in the past. For example, the forthcoming IPCC AR5 notes climate models have predicted that in a warmer climate, increased evaporation will increase low cloud thickness, vertical, and horizontal extent, all of which increases reflection of sunlight [albedo], cools the planet, and acts as a negative feedback.

"The modelled response of low clouds does not appear to be dominated by a single feedback mechanism, but rather the net effect of several potentially competing mechanisms as elucidated in LES and GCM sensitivity studies (e.g., Zhang and Bretherton, 2008; Blossey et al., 2013; Bretherton et al., 2013). Starting with some proposed negative feedback mechanisms, it has been argued that in a warmer climate, low clouds will be: (i) horizontally more extensive, because changes in the lapse rate of temperature also modify the lower tropospheric stability (Miller, 1997); (ii) optically thicker, because adiabatic ascent is accompanied by a larger condensation rate (Somerville and Remer, 1984); and (iii) vertically more extensive, in response to a weakening of the tropical overturning circulation (Caldwell and Bretherton, 2009)." - AR5 draft pg 7-20

The authors base the claim upon their computer model which allegedly overturns the prior 'settled science' on clouds and thereby proclaims the globe will warm 4C by 2100.

"They report in Nature that updraughts of water vapour can rise 15 kms to form high clouds that produce heavy rains, or the vapour can rise just a few kilometers before coming back to the surface without forming rain clouds. When this happens the process actually reduces the overall cloud cover because it dessicates the clouds above: it draws away water vapour from the higher regions in a process called convective mixing. [see New paper finds IPCC climate models don't realistically simulate convection and thus convective mixing]

In a warmer world there may be fewer clouds – and less of a cooling effectImage: Fir0002 at English Wikipedia

By Tim Radford

One of the great unknowns of climate science is what effect clouds have in accelerating or slowing warming. A new study sheds a disturbing light on their possible impact.

LONDON, 31 December – Australian and French scientists believe they have cracked one of the great puzzles of climate change and arrived at a more accurate prediction of future temperatures.

The news is not good, according to Steven Sherwood of Australia’s Centre for Excellence for Climate System Science at the University of New South Wales. If carbon emissions are not reduced, then by 2100 the world will have warmed by 4°C.

This figure does not, at first, sound high: researchers have been warning for 20 years on the basis of computer models that under the notorious business-as-usual scenario in which everybody goes on burning coal and oil, then as carbon dioxide levels double, global temperatures could rise by between 1.5°C and 4.5°C.

Pessimists could cite one extreme, optimists the other: the range of uncertainty was a recognition that there were still some big unknowns in the machinery of climate, and one of those unknowns was the behaviour of the clouds in a warmer world.

More warmth means more evaporation, more vapour could mean more clouds. Low-level clouds reflect sunlight back into space, and help cool the climate a bit. This is what engineers call negative feedback.

Drying the clouds

But if more water vapour actually led to less cloud, then more sunlight would reach the surface and the world would warm even more: positive feedback would be in play. Climate models cater for such possibilities, but cannot choose between them.

What Sherwood and his colleagues from Pierre and Marie Curie University in Paris did was to start with some real-world observations of what happens when water vapour gets into the atmosphere.

They report in Nature that updraughts of water vapour can rise 15 kms to form high clouds that produce heavy rains, or the vapour can rise just a few kilometers before coming back to the surface without forming rain clouds.

When this happens the process actually reduces the overall cloud cover because it dessicates the clouds above: it draws away water vapour from the higher regions in a process called convective mixing.

Profound effects in prospect

So the next step was to feed the new understanding into computer simulations. These then showed that climate cycles could develop that would take vapour to a wider range of heights in the atmosphere, with the consequence that fewer clouds would form as climate warms.

If so – and other climate scientists will have their own arguments with the findings – then as carbon dioxide levels double, which they will do in the next 50 years or so, the average planetary temperatures will increase by a colossal 4°C.

Governments have expressed the wish – but not so far taken the necessary action – to contain planetary temperatures to a rise of no more than 2°C. If Sherwood and colleagues are right, they will not get their wish. And the process will go on. The temperatures will continue to soar beyond 2100, to reach an additional 8°C by 2200.

“Climate skeptics like to criticise climate models for getting things wrong, and we are the first to admit they are not perfect, but what we are finding is that the mistakes are being made by those models that predict less warming, not those that predict more”, said Professor Sherwood.

“Rises in global average temperatures of this magnitude will have profound impacts on the world and the economies of many countries if we don’t urgently curb our emissions.” – Climate News Network

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Abstract

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

3 comments:

During the late 20th century warming period total global cloudiness did decrease and it is good that they have acknowledged that at last.

Their assumption that the cloudiness changes were driven by CO2 changes is false.

As per my New Climate Model that reduction was due to an active sun pulling the global air circulation poleward to cause more zonal jet stream tracks and allowing more solar energy into the oceans to warm up the climate system.

Since about 2000 the process has reversed with more meridional jets and a general shift of the global air circulation back towards the equator.

That reversal of trend could not have occurred if CO2 were the driver since CO2 emissions have increased since 2000.